It is important to find binary surfactant systems having synergistic interactions (negative value of the interaction parameter) since it means that a binary surfactant mixture composition exists in these systems, having lower critical micelle concentration value than more hydrophobic mixture component. In this work, influence of hydrophobic segment length of homologues Tweens in mixed micelles with Triton X-100 on the interaction parameter value (beta(ij)) and on the excess Gibbs energy (G(E)) was studied. Also, influence of temperature on the stability of investigated mixed micelles has been investigated (298.15-328.15 K). The critical micelle concentration values of pure surfactants and their binary mixtures were determined spectrofluorimetrically using pyrene as a probe molecule. It was found for every investigated temperature that increase in length of saturated aliphatic side chain of a polysorbate increases the stability of investigated mixed micelles (beta(ij) < 0 and G(E) < 0) compared to the ideal mixed micelles. Presence of the C9 double bond in oleic acid residue of Tween 80 reduces synergistic effect of this polysorbate compared to Tween 60 which contains hydrophobic segment with the same number of C atoms but without olefin bond. The excess free enthalpy below the 298.15 K is determined with entropic contribution, but above 298.15 K with enthalpic contribution. (C) 2014 Elsevier B.V. All rights reserved.